One of the more important systems on any vehicle is its braking system. Clearly, failure of the braking system on a vehicle creates a high probability of an accident causing substantiel property damage and injury. Accordingly, modern automobiles are typically provided with redundant braking systems to ensure that at least partial braking capacity remains should the primary system fail.
A reliable brake system is even more critical on an aircraft, particularly large commercial jet aircraft carrying hundreds of passengers and landing at speeds of several hundred miles per hour. Failure of the brake system on an aircraft may cause the plane to overshoot the end of the runway, or to collide with other taxining aircraft or ground structures, resulting in damage and possible loss of life. Thus, it is important that the braking systems installed on aircraft be designed to greatly reduce the likelihood of even a partial brake failure. Unlike either the manual or power assisted brakes of an automobile, large aircraft brakes are actuated by pressurized fluid provided by a hydraulic pump in response to brake pedal force. The brake pedal controls a metering valve to modulate the pressure applied to the brakes. A fully redundant braking system on an aircraft includes both multiple sources of pressurized fluid and multiple metering valves to ensure that full braking capacity is always available.
A large commercial jetliner such as the Boeing 747 includes a primary, a secondary, and a reserve source of pressurized hydraulic fluid for actuating the brakes. The brake system for each landing gear includes both "normal" and reserve metering valves and normal and reserve anti-skid valves. The flight crew monitors the hydraulic fluid pressure in the brake system and manually switches to the secondary source system if the primary system pressure drops below a predetermined level. Should the quantity of hydraulic fluid in the primary or secondary brake system fall below a critical level, the crew may switch to the reserve source of pressurized brake fluid and to the reserve metering and anti-skid valves, since it is presumed that there is a leak either in the primary/secondary source supply lines or in the normal metering and anti-skid valves. Thus, one of the flight crew members must be responsible for monitoring the status of the brake systems at critical times. When three crew members are available this is not a problem. However, it has been proposed that only a two-man flight crew be provided. Since both crew members must devote their attention to other systems on the aircraft prior to and during landing, no one will be available to continually monitor the condition of the brake system. The current, manual braking source select system will thus be unacceptable for use on a larger aircraft having only the proposed two-man flight crew.
Any automatic brake source selection system must be extremely reliable, and capable of functioning even if there is a loss of electrical power, or failure of any single component in the system. An unattended brake source select system should also automatically brake the wheels upon retraction of the landing gear. Should the primary source of brake pressure fail, the brake source select system should automatically revert to a source having the next lower priority, but should alert the crew of the aircraft that the primary system has failed by lighting a warning light on an annunciator display panel. Furthermore, to minimize costs, the automatic source select system should require minimal alteration of the braking system of an aircraft on which it is retrofitted. The present invention meets these requirements better than a number of alternative approaches that were considered, and its advantages will be apparent from the attached drawings and the description of the preferred embodiments that follow.